A variety of agents are presently used to combat viral infection. These agents include interferon, which is a naturally-occurring protein having some efficacy in combat of certain selected viral diseases. In addition, agents such as AZT are used in the combat of an immunodeficiency disease, referred to commonly as AIDS, caused by the virus HIV-1.
Drug and Market Development, Vol 3. No. 9, pp. 174-180 (Feb. 15, 1993), describes antiviral drug development.
It states:
The difficulties encountered in drug treatment of most infections pale when compared to viral infections. For example, it is at least theoretically (and often in practice) possible to attack a bacterium without harming the host. Unlike bacteria however, viruses replicate inside cells and utilize cellular machinery of the host for replication. As a result, development of antiviral therapeutics often represents a compromise between preferable killing, or at least arresting replication of, the virus, and not harming the host, or at worst, doing only minimal damage which can be justified by the potential gain.
It states that viral specific events can be targeted including:
Virus attachment to cell membranes and penetration in cells;
Virus uncoating;
Virus nucleic acid synthesis;
Viral protein synthesis and maturation; and
Assembly and release of infectious particles.
Specifically with regard to viral protein synthesis the authors state:
In contrast to nucleic acid synthesis, viral protein synthesis utilizes host ribosomes (ribosomes are cell structures essential for translation of mRNA into protein) and mostly host-derived supplementary factors. As a result, protein synthesis inhibitors, in general, are as likely to exhibit host toxicity as they are to exert antiviral effects. Antisense oligonucleotides, however, may be of value in specifically inhibiting viral protein synthesis. Briefly, antisense oligonucleotides are short DNA fragments that are complementary to mRNA (sense strands) and can prevent mRNA-directed protein synthesis by binding to mRNA. RNA molecules have also been constructed to contain sequences complementary to those of sense DNA strands (and their corresponding mRNA). Although antisense constructs have been shown to inhibit viral protein synthesis in vitro, their effectiveness in vivo has not yet been conclusively demonstrated. Among others, current challenges for oligonucleotide therapeutics include delivery to virus-infected cells, the stability of such molecules in vivo and distribution throughout the body.
Ribosome inactivators represent another approach for viral protein synthesis inhibition. GLQ223 (Genelabs; Redwood City, Calif.) is a ribosome inactivator undergoing clinical testing (GLQ223 is a purified preparation of trichosanthin (cucumber plant derivative)). A ribosome inactivator would interfere with cellular translation machinery, effectively preventing generation of new viral proteins.
Sonenburg, 2 The New Biologist 402, 1990 describes virus host interactions at the level of initiation of translation and states that two initiation factors eIF-2 and eIF-4F play significant roles in a number of virus host interactions. He states xe2x80x9c[a]n understanding of the mechanisms responsible for these virus-host interactions is of great signifigance for future therapeutic approaches to viral disease.xe2x80x9d
The present invention relates to methods for screening for agents which are effective in inhibiting the translational system used by a virus during infection of a host cell. The screening method utilizes a protocol in which potentially useful agents are brought into contact with appropriate macromolecular sequences, e.g., viral nucleic acid sequences or relevant protein sequences, in order to determine whether those agents can specifically inhibit use of those sequences. Viruses use a variety of methods for taking over a host translational system, and it is these methods that can be specifically targeted by methods of the present invention. Once isolated, the viral specific agents can be formulated in therapeutic products (or even prophylactic products) in pharmaceutically acceptable formulations, and used for specific treatment of viral disease with little or no effect on uninfected virus host cells.
Specifically, in one aspect, applicant provides a screening method in which a target virus nucleic acid sequence or domain responsible for preferential translation of viral RNA over host RNA is used in a selection protocol. While several specific examples of such viral nucleic acid sequences or domains are provided below in the form of IRES elements, 5xe2x80x2-untranslated regions containing specific viral sequences, and upstream open-reading frames containing such sequences, these are used only to exemplify a general method by which other virus nucleic acid sequences can be used in such protocols. Use of any one of these virus nucleic acid sequences within a cell translation system provides a means by which anti-viral agents can be discovered.
Applicant notes that the claimed method does not include targeting of agents to viral sequences involved in frame shifting (which is not a target nucleic acid that is preferentially translated as defined herein), such as described by Dinman and Wickner, 66 J. Virol. 3669, 1992; Jacke et al., 331 Nature 280, 1988; Wilson et al., 55 Cell 1159, 1988; Inglis and Brierly, WO 90/14422; and Goodchild and Zamecnik, WO 87/07300.
Any agent which binds to such viral nucleic acid and/or which causes a significant reduction in translation of viral message is potentially useful in the present invention. Such agents can be screened to ensure that they are specific to viral translation systems and have no effect on uninfected host cell translation systems such that the agent can be used in a therapeutic or prophylactic manner. If such agents have some effect on host cell systems they may still be useful in therapeutic treatment, particularly in those diseases which are life threatening, such as HIV-1 infection.
Such agents may interact either directly with the target viral nucleic acid, for example, by hybridization with the nucleic acid, e.g., antisense RNA or DNA, or may bind or interact with other components of the viral translation system (i.e., those host and/or viral components whether nucleic acid and/or protein which allow translation of viral mRNA to occur in vivo), such as proteins used by the virus to promote translation of its RNA, rather than host RNA involved in that system, e.g., antibodies. Additionally, agents may include any nucleic acid molecule which binds to viral or cellular components which otherwise would partake in preferential viral nucleic acid translation, but upon binding said nucleic acid molecule become unable to be preferentially translated. However, while antisense nucleic acid and antibodies may exemplify aspects of the present invention, applicant is particularly concerned with identification of agents of low molecular weight (less than 10,000, preferably less than 5,000, and most preferably less than 1,000), which can be more readily formulated as useful antiviral agents. Thus, in a preferred embodiment, the invention features such low molecular weight agents, and not antisense molecules or antibodies.
Thus, in a first aspect the invention features a method for screening for an antiviral agent. The method includes providing a target viral translation nucleic acid sequence which allows preferential translation of a viral RNA compared to a host RNA under virus infection conditions. The method may involve a simple assay to detect binding of an agent to this nucleic acid Preferably, however, the target viral translation nucleic acid sequence is translationally linked to RNA encoding a reporter polypeptide. The method then further includes contacting the target viral translation nucleic acid squence with a potential antiviral agent under conditions which allow synthesis of the reporter polypeptide in the absence of the agent. The method finally includes determining whether the agent reduces the level of translation of the reporter polypeptide. Any agent which does reduce this level is potentially a useful antiviral agent.
Specifically, the method involves determining whether a potential agent interacts with a virus or cellular component which allows or prevents preferential translation of a virus RNA compared to a host RNA under virus infection conditions; and determining whether any interaction of the agent with the component reduces the level of translation of a RNA of the virus.
By xe2x80x9cscreeningxe2x80x9d is preferably meant a process in which a large number of potentially useful agents are processed in the method of this invention. It is generally a process distinct from a single experiment in which a single agent is studied in detail to determine its method of action.
By target viral translation nucleic acid sequence is meant any nucleic acid which allows preferential translation of translationally associated RNA under viral infection conditions. Such nucleic acid is exemplified by IRES elements which allow cap-independent translation of associated ribonucleic acid, and 5xe2x80x2 untranslated regions of influenza virus RNA which allow preferential cap-dependant translation of associated RNA.
By preferential translation is meant that the RNA is translated at a higher rate or with higher yield of protein than host cell RNA under virus-infection conditions. In addition, the host cell RNA may be translated at a slower rate or with lower protein yield than in non-infected conditions. Such preferential translation cn be readily detected as described below. In the case of most viruses, preferential expression of viral proteins means that synthesis of viral proteins represents at least 50% of total de novo protein synthesis, as may be detected, for example, by pulse-labeling experiments in viral-infected cells. In such cases, viral proteins may usually be distinguished as major bands when labeled proteins are separated by gel electrophoresis. In the case of retroviruses, preferential expression of viral proteins means that the level of viral proteins synthesized increases disproportionately beyond the level of viral RNA synthesized (Cullen, Cell 46: 973, 1986). Such a disproportionate increase can be detected by quantitating levels of viral RNA and protein synthesis in infected cells by, for example, Northern blotting and nuclease protection assays for RNA synthesis and immunoprecipitations and gel electrophoresis for labeled proteins.
By virus infection conditions is simply meant conditions within a host cell after infection with the target virus such that the viral translation system is operative. Such a viral translation system will usually include host cell proteins, nucleic acids and other components.
By reporter polypeptide is simply meant a peptide which is readily detectable, either by providing a colorimetric signal under certain environmental conditions or some other signal well known to those of ordinary skill in the art, as described below.
In preferred embodiments, the component is a protein or a nucleic acid; the component is virus encoded or host cell encoded; the component is a macromolecule selected from an RNA sequence domain, a DNA sequence domain, an initiation factor, and elongation factor, a termination factor, a transcription factor, a ribosomal protein, a glycosylase, a deglycosylase, a prenylating and deprenylating enzyme, a transferase, a polymerase, a synthetase, an ADP ribosylating enzyme, an ADP ribosylase, a kinase, a lipase, a myristylating or demyristylating enzyme, a phosphorylase, a protease, a rRNA, a tRNA, a ribonuclease, and a deoxyribonuclease; the viral translation signal nucleic acid sequence is selected from the group consisting of IRES elements, 5xe2x80x2 or 3xe2x80x2 untranslated regions, and upstream open reading frames, or any other viral target translation nucleic acid that affords preferential translation of viral mRNA over host cell mRNA when the host cells are infected by the virus; and the virus from which that signal is selected is chosen from the picornavirus family, Hepatitis viruses A, B, and C, influenza virus, HIV, Herpes virus, and cytomegalo-virus.
In other preferred embodiments, the sequence domain is translationally linked to RNA encoding a reporter polypeptide, and the second determining step includes determining whether the agent alters the level of translation of the reporter polypeptide; the component is a protein or a polypeptide, and the determining steps include providing the component in a translation mixture with RNA encoding a reporter polypeptide, and determining whether the agent alters expression of the reporter polypeptide in the mix.
In more preferred embodiments, the method further includes determining whether an agent active in the above method has little or no effect on the translational machinery of an uninfected viral host cell, and further determining whether the agent is active under in vivo conditions. Such agents are then formulated in a pharmaceutically acceptable buffer.
By pharmaceutically acceptable buffer is meant any buffer which can be used in a pharmaceutical composition prepared for storage and subsequent administration, which comprise a pharmaceutically effective amount of an agent as described herein in a pharmaceutically acceptable carrier or diluent. Acceptable carriers or diluents for therapeutic use are well known in the pharmaceutical art, and are described, for example, in Remington""s Pharmaceutical Sciences, Mack Publishing Co. (A. R. Gennaro edit. 1985). Preservatives, stabilizers, dyes and even flavoring agents may be provided in the pharmaceutical composition. For example, sodium benzoate, sorbic acid and esters of p-hydroxybenzoic acid may be added as preservatives. Id. at 1449. In addition, antioxidants and suspending agents may be used. Id.
In a second aspect, the invention features a method for treating a subject infected with a virus having a viral translation signal nucleic acid sequence, by administering to that subject a therapeutically effective amount of an antiviral agent able to selectively block translation of viral RNA naturally linked to the viral translation signal nucleic acid sequence.
By xe2x80x9ctherapeutically effective amountxe2x80x9d is meant an amount that relieves (to some extent) one or more symptoms of the disease or condition in the patient. Additionally, by xe2x80x9ctherapeutically effective amountxe2x80x9d is meant an amount that returns to normal, either partially or completely, physiological or biochemical parameters associated with or causative of a viral disease. Generally, it is an amount between about 1 nmole and 1 xcexcmole of the molecule, dependent on its ECSU and on the age, size, and disease associated with the patient.
In a third related aspect, the invention features novel antiviral agents discovered by the methods described above. It also includes novel pharmaceutical compositions which include antiviral agents, discovered as described above, and formulated in pharmaceutically acceptable formulations.
In a fourth aspect, the invention features the use of nucleic acid constructs containing isolated viral nucleic acid translationally linked to a reporter-encoding sequence to discover antiviral agents, and kits for use of these constructs in antiviral agent screening methods.
In a fifth aspect, the present invention features a screening method for antiviral agents active at modulating the activity of other, non-nucleic acid, macro-molecules involved in the viral mRNA translation system. For example, the method for screening agents includes identifying those effective at inhibiting macromolecules that interfere with the activity of such macromolecules, e.g., agents which allow the p68 kinase in a cell to exhibit its activity. The invention also features a method of employing such agents for inhibiting replication of virus in eukaryotic host cells.
Thus, the invention includes a method of inhibiting viral replication in a host eukaryotic cell, e.g., where the virus produces a viral inhibitor which interferes with the activation of a host-cell interferon-induced, double-stranded RNA-activated protein kinase. The method includes administering to the cells, an agent able to block the effect of the viral inhibitor in interfering with the activation of the protein kinase.
In a related aspect, the invention features a virus which produces a viral inhibitor able to block binding of double-stranded RNA to the protein kinase, and the agent administered is one able to block the binding of the viral inhibitor to the protein kinase.
The agent may be selected, for example, by forming a mixture composed of protein kinase, the viral inhibitor, and the agent, incubating the components of the binding mixture under conditions effective to bind the protein kinase to the viral inhibitor, in the absence of the agent, and examining the mixture for the presence of binding of the protein kinase to the viral inhibitor, to determine whether the presence of the test agent has inhibited binding the protein kinase to the viral inhibitor.
Alternatively, the agent may be selected by forming a mixture composed of protein kinase, the viral inhibitor, and the agent, incubating the components of the mixture under conditions effective to autophosphorylate the protein kinase in the absence of the viral inhibitor, examining the mixture for the presence of protein kinase activity, and selecting the agent if it is able to prevent inhibition of protein kinase activity.
In specific examples, where the virus is an adenovirus, and the viral inhibitor is a VAI RNA molecule (also known as VA 1 and VARNAt); the virus is human immunodeficiency virus (HIV), and the viral inhibitor is a TAR region of the HIV genome; and the virus is an Epstein-Barr virus, and the viral inhibitor is an EBER-1 RNA.
In another related aspect, the viral inhibitor is effective to activate a host-cell p58 protein which is able, in activated form, to block the activation of the protein kinase, or to block the activity of already activated protein kinase, and the agent is one which blocks the interaction of the viral inhibitor through p58 protein on the kinase. The agent may be selected, for example, by forming a mixture composed of protein kinase, the p58 protein (an active form), and the agent, and then incubating the components of the mixture under conditions effective to autophosphorylate the protein kinase, when the p58 protein is absent, examining the mixture for the presence of protein kinase activity, and selecting the agent if it is able to reduce inhibition of protein kinase activity, when p58 is present.
In another related aspect, the invention includes a method for screening agents effective to inhibit viral replication in a host eukaryotic cell, where the virus is one able to produce a viral inhibitor which interferes with the activation of the host-cell interferon-induced, double-stranded RNA-activated protein kinase. The method includes incubating a mixture containing the protein kinase, the viral inhibitor, and the agent to be tested, under conditions effective to cause viral inhibitor interference with the activation of the protein kinase, and examining in mixture for such interference.
The method of this invention can also be used for screening an agent effective to inhibit replication in a host cell of a virus which produces a viral inhibitor capable of binding to the protein kinase, to inhibit binding of double-stranded RNA to the protein kinase. In this method, the mixture is incubated under conditions effective to bind the viral inhibitor to the protein kinase, and the mixture is examined for binding of the viral inhibitor to the protein kinase. The incubating may be carried out, for example, in solution phase, and the examining step includes passing the mixture through a filter which retains the viral inhibitor only when the inhibitor is bound to the protein kinase. Alternatively, the protein kinase may be bound to a solid support, the viral inhibitor labeled with a reporter, and the examining step performed by measuring the amount of reporter bound to the solid support. In addition, the incubating may be carried out under conditions in which the protein kinase is autophosphorylated, in the absence of binding to the viral inhibitor, and the examining step performed by determining the extent of phosphorylation of the p68 kinase.
In another related aspect, the method of this invention is used for screening agents effective in blocking viral replication of a virus which produces an viral inhibitor effective to activate a p58 host-cell protein which in activated form is effective to block autophosphorylation of the protein kinase or to block activity of the phosphorylated kinase. Here the mixture formed includes the p58 host-cell protein, the incubating step is carried out under conditions in which the protein kinase would be autophosphorylated in the absence of p58, and the mixture is examined for reduction of inhibition of protein kinase activity.
In still another aspect, the protein kinase and viral inhibitor art expressed in a yeast cell which is constructed to increase the expression of a marker protein in the presence of activated protein kinase, and the yeast cells are examined for increased expression of the marker protein. This aspect concerns use of a yeast cell in screening agents effective to inhibit viral replication in a host eukaryotic cell, where the virus is able to produce a viral inhibitor which interferes with the activation of the host-cell interferon-induced, double-stranded RNA-activated protein kinase. The cell includes (a) an expressible gene encoding a mammalian interferon-induced, double-stranded RNA-activated protein kinase, (b) a reporter gene whose expression in increased by activation of the protein kinase, and (c) a viral gene for producing a viral inhibitor able to block activation of the protein kinase.
In yet other preferred embodiments, the method of this invention includes forming a protein translation mixture which includes (i) a viral mRNA construct, the mRNA construct comprising (a) an internal ribosome entry site (IRES) region and downstream of the IRES region, a first reporter protein coding region, (ii) ribosomes, and (iii) an agent to be tested, incubating the components of the translation mixture under conditions effective to produce from the first reporter protein coding region a reporter protein, and examining the mixture for the presence of reporter protein produced by such translation mixture, and the agent is a useful anti virus agent if the reporter protein produced in the presence of the test agent is less than an amount of reporter protein produced in the absence of the test agent.
Preferably, the IRES region is derived from a picornavirus IRES region sequence; the IRES sequence is selected from the group consisting of an enterovirus, rhinovirus, cardiovirus, and aphthovirus IRES sequence; the IRES region is selected from the group consisting of an hepatitis A virus IRES sequence, an hepatitis B virus sequence and an hepatitis C virus IRES sequence; the protein translation mixture is a cell-free extract; the 5xe2x80x2-end of the viral mRNA construct includes a eukaryotic mRNA 5xe2x80x2-terminal cap and untranslated region (UTR) and downstream of the cap and UTR region, a second reporter protein; and the translation mixture is contained in a cell.
In another example, the method includes forming a binding mixture comprising a cellular or viral translation initiation protein, an IRES element ribonucleotide sequence, and an agent to be tested, incubating the components of the binding mixture under conditions effective to bind the initiation protein to the IRES element, and examining the mixture for the presence of binding of the initiation protein to the IRES element. The agent is a useful antivirus agent if the extent of binding of the initiation protein to the IRES element is less than that observed in the absence of the agent.
Preferably, the cellular or viral translation initiation protein is selected from the group consisting of p52 and p57; the cellular or viral translation initiation protein is bound to a solid support, the IRES element is labeled with a reporter, and the examining includes measuring the amount of reporter bound to tie solid support; the IRES element RNA is bound to a solid support, the cellular or viral translation initiation protein is labeled with a reporter, and the examining includes measuring the amount of reporter bound to the solid support; a terminal region of the IRES element is bound to a complementary DNA sequence, and the DNA sequence is linked to the solid support; and the method further includes the step, after the incubating step, of adding to the incubation mixture an RNAase capable of cleaving free RNA but not protein bound RNA, and the binding of the initiation protein to the IRES element is detected by the presence in the mixture of uncleaved IRES element RNA.
In one example, the examining includes subjecting the mixture to a gel-shift electrophoresis assay.
In still other preferred embodiments, the incubating is carried out in solution phase, and the examining includes passing the mixture through a filter which retains the IRES element only when the element is bound to the cellular or viral translation initiation protein.
In a related aspect, the agent is effective to inhibit viral replication in a host eukaryotic cell, where the virus produces an inhibitor which interferes with the activation or activity of the host-cell interferon-induced, double-stranded RNA-activated protein kinase, and the screening method includes incubating a mixture containing the protein kinase, the inhibitor, and the agent to be tested under conditions effective to cause inhibitor interference with the activation or activity of the protein kinase, and examining the mixture for such interference; or the agent is effective to inhibit viral replication in a host eukaryotic cell, where the host cell produces an inhibitor which interferes with the activation of the host-cell interferon-induced, double-stranded RNA-activated protein kinase, and the method includes incubating a mixture containing the protein kinase, the inhibitor, and the agent to be tested under conditions effective to cause inhibitor interference with the activation of the protein kinase, and examining the mixture for such interference.
Preferably, the method is for use in screening an agent effective to inhibit replication in a host cell of a virus which produces an inhibitor able to bind to the protein kinase, to interfere with the activation of the protein kinase by double-stranded RNA, and the incubating includes incubating the protein kinase, viral inhibitor, and agent under conditions effective to bind the inhibitor to the protein kinase, and the examining includes examining the protein kinase for bound inhibitor; or the incubating is carried out in solution phase, and the examining includes passing the protein kinase, viral inhibitor, and test agent through a filter which retains the inhibitor only when the inhibitor is bound to the protein kinase; or the protein kinase is bound to a solid support, the inhibitor is labeled with a reporter, and the examining includes measuring the amount of reporter bound to the solid support; or the incubating is carried out under conditions in which the protein kinase is autophosphorylated, in the absence of binding to the viral inhibitor, and the examining includes determining the extent of phosphorylation of the p68 kinase; or the method is for use in screening agents effective in blocking viral replication of a virus which produces an inhibitor effective to activate a p58 host-cell protein which in activated form is effective to block activity or activation of the protein kinase, and the mixture formed includes the p58 host-cell protein, the incubating is carried out under conditions in which the protein kinase is activated in the absence of p58, and the examining includes examining the mixture for inhibition of protein kinase activity.
In a preferred embodiment, the protein kinase and inhibitor are expressed in a yeast cell which is constructed to increase the expression of a reporter protein in the presence of activated protein kinase, and the examining includes examining the yeast cells for increased expression of the reporter protein; and the reporter protein is fused GCN4/xcex2-gal protein.
In another aspect, the invention features a yeast cell for use in screening agents effective to inhibit viral replication in a host eukaryotic cell, where the virus produces a viral inhibitor which interferes with the activation of the host-cell interferon-induced, double-stranded RNA-activated protein kinase. The cell includes (a) an expressed gene encoding a mammalian interferon-induced, double-stranded RNA-activated protein kinase, (b) a reporter gene whose expression in increased by activation of the protein kinase, and (c) a viral gene for producing a viral inhibitor able to block activation of the protein kinase.
Preferably, the reporter gene is a fused GCN4/xcex2-gal gene.
In a related aspect, the yeast cell for use in screening agents effective to inhibit viral replication in a host eukaryotic cell, where the virus activates or induces a cellular protein to interfere with the activation of the host-cell interferon-induced, double-stranded RNA-activated protein kinase, includes the components (a) and (b) above and (c) a gene encoding a protein which blocks activation of a cellular protein.
Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof, and from the claims.